This invention relates to a method of removal and recovery of oil and grease from wastewater, in particular, to a method using spinel ferrite powder, which possesses strong surface activity but is insoluble in weak acid or alkaline, whereby the powder is added to the waste to adsorb the oil and grease contained in waste and then to separate solid powder and liquid by an external magnetic field.
There are two types of oil and grease which cause water pollution: one is animal and plant fat, and the other is mineral oil, such as petroleum oil and refined oil. Owing to their lower densities, oil or grease will not mix well with water and therefore becomes an immiscible film suspended on the water surface. This film will inhibit the dissolving of oxygen in water and reduce the dissolving capacity of oxygen in water. The result will result in serious environmental problems.
Wastewater containing oil and grease is often effused from petroleum refineries and chemical plants. Generally speaking, the mixture of oil and water can be classified as suspended oil, dispersed oil, emulsified oil, etc. The suspended oil can be removed through gravity separation methods, such as American Petroleum Institute separator (API), Corrugated-Plates Interceptor (CPI) and Parallel-Plates Interceptor (PPI). When free oil or oil moiety suffers mechanical impact, the oil will either form particles and emulsify; or will meet a surface activator and become a stable emulsifion which cannot be treated through gravity separation methods. In addition, water soluble machine oil or cut oil is now used in several plants to enhance the technology of mechanical processes. Owing to the chemical effects of a surface activator, said oil waste becomes emulsified and the size of the dispersed oil particle is in the range of several ums, does not possess upward buoyancy and will stably disperse in water. Animal and plant oil wastes from a food cannery easily became emulsified and cannot be handled easily.
There currently exists several separation methods of oil and wastewater, and treatments of oil and grease decomposition which are shown in Table 1. The methods are used to treat water soluble oil and grease containing adsorption of membrane, biological, active carbon, etc. Owing to technical problems and economic reasons, the practical methods are limited.
TABLE 1 ______________________________________ Oil-removal processes summary Process Advantages Disadvantages ______________________________________ Gravity separation Economical Limited efficiency (API separators, Simple Susceptible to CPI separators, operation weather conditions PPI separators) Removes little or no soluble oil Limited removal of emulsified oil Air flotation Handles high Chemical sludge (DAF & IAF) solids disposal when Reliable coagulants are process used (DAF only) (handles Requires chemicals shock load) Chemical coagulation- Handles high Excessive Chemical Flocculation and solids sludge produced sedimentation Not cost-effective Coalescers (plate, High potential Cannot handle high fibrous and loose efficiency solids due to media) fouling Membrane processes Soluble oil Low flux rates removal Membrane fouling and questionable membrane life Not demonstrated as a practical process for oil and grease removal Biological Removes Solids carryover soluble oil Prone to upset Relatively Pretreatment prere- high toler- quisite ance for oil and grease Carbon adsorption Removes solu- Expensive ble oil Regeneration High poten- required tial Requires extensive efficiency pretreatment Full-scale oper- ation not proven in refinery Energy-intensive ______________________________________
In addition, ferrites or other ferromagnetic materials in conjunction with hydrophobic material has been used to separate oil from water. Adsorption appears to the primary mechanism operation in the removal of oil by ferrites or other ferromagnetic materials. Conventionally, polystyrene beads were coated or impregnated with ferrite or other ferromagnetic material. When these beads were spread in water containing oil, the polystyrene adsorbed the oil without dissolving or otherwise mixing with the aqueous phase. The beads were subsequently removed magnetically by means of the ferrite. Finally, the oil can be removed by incineration of the ferrite used. In fact, the temperature of incineration is about 400.degree.-500.degree. C. and will somewhat influence the magnetism of the ferrites or other ferromagnetic materials. Hence, it is difficult for these materials to be reused.
The present invention is to provide a highly efficient, low cost, new economical technology for treating and recovering oil and grease from wastewater. The method disclosed in the present invention not only treats suspended oil, dispersed oil and emulsified oil in wastewater by solving some technological difficulties, but also recovers the oil and grease for re-use to meet the purposes of cost saving and waste reduction. More importantly, the recovered ferrite or other ferromagnetic material does not suffer high temperature alteration as occurs in the conventional methods.